How to use the @optimizely/js-sdk-utils.keyBy function in @optimizely/js-sdk-utils

// these must be constructed separately
    variations: Variation[]
    group?: Group
  }) {
    this._id = id
    this._status = status
    this._key = key
    this._layerId = layerId
    this._audienceIds = audienceIds
    this._variations = variations
    this._forcedVariations = forcedVariations
    this._group = group

    this.trafficAllocation = new OptimizelyTrafficAllocation({
      allocations: trafficAllocation,
      entityIdMap: keyBy(variations, variation => variation.id),
    })
  }